Titanium dioxide pigments



Patented June 19, 1945 TITANIUM DIOXIDE PIGMENTS Durant W. Robertson, Montclair, N. 3., asslgnor to National Lead Company, New York, N. Y., a corporation of New Jersey No Drawing.

14 Claims.

' This invention relates to titanium dioxide pigments and to methods for improving the technical properties of such pigments. For instance, titanium dioxide pigments prepared in accordance with the present invention when incorporated in surface coating compositions are rendered more resistant to weathering, that is to say, exhibit enhanced gloss retention and a reduced tendency toward chalking, and, at the same time are materially improved in their resistance to discoloration in the light.

The poor weathering characteristics and the discoloration on exposure to light have heretofore restricted the use of titanium dioxide pigments in coating compositions, and the superior pigmenting power of said pigments could therefore not be utilized to the fullest extent in the formulation of paints and other coating materials for exterior service.

The principal object of the present invention, among others, is to provide methods for the preparation of titanium dioxide pigments which shall have improved gloss retention, enhanced resistance to chalking, improved color stability, and shall be substantially free from any tendency to promote the fading of organic coloring material. This and other objects of the present invention will become apparent as this description unfolds.

In its broadest conception, the present invention comprises a treatment of a previously prepared titanium dioxide pigment with a substantially water insoluble, white silicate to bring about an intimate and uniform association, possibly a coating, of the particles of titanium dioxide pigment and the insoluble silicate. It is essential that this intimate association of pigment particles and silicate be carried out in a moist state. This may be accomplished (a) by suspending the pigment particles in aqueous media and forming the insoluble silicate, in situ, Or (b) by intimately associating the pigment particles with gelatinous, non-coagulated, hydrated, silicate gels. For purposes of operation, the first method will be preferred in most applications of the present invention.

The substantially water insoluble, white silicates which have been found most useful in the practice of the invention are those of metals of the second, third and fourth groups of the periodic system which may be formed by the reaction between a water soluble silicate and a water soluble compound of a metal of these groups. For instance, the silicates of magnesium and zinc, representative of metals of group '11, those of aluminum and yttrium, representative of metals Application October 31, 1939, Serial No. 302,116

of group III, and those of zirconium and cerium, representative of metals of group IV are especially useful. 7

When the insoluble silicate is to be formed, :in situ, the general method of practicing the invention will be as follows:

A water soluble metal compound the silicate of which is white and substantially water-insoluble,

preferably a compound of a metal of the second, third, or fourth group is added to an aqueous slurry of a previously prepared titanium dioxide pigment. The slurry may be prepared by any suitable known means such as milling, grinding, or vigorously stirring the pigment particles into the aqueous medium. Known dispersing agents may be employed to assist in the dispersion of the pigment particles, if desired. The water soluble metal compound may be added to the slurry either as a water solution thereof or as a pulverized solid while properly agitating the mass to insure good distribution of the soluble metal compound.

A water soluble silicate is now added to this slurry whereby the silicate compound is precipitated and incorporated with the titanium dioxide pigment, the slurry then being filtered and washed and the pigment dried and further handled in a manner customary for the finishing of dried pigments. Alternatively, a water soluble silicate may be added to the pigment slurry whereby the pigment is more completelydispersed and whereby, if so desired, any gritty material and agglomerates more easily may be eliminated by hydroseparation or by other means prior to the precipitation of the diiflcultly soluble silicate by the addition of the water soluble metal compound. The pigment slurry also may be advantageously treated prior to the silicate treatment in a colloid or pebble mill in order to disintegrate gritty materials and agglomera'tes,

which may be present in the pigment.

The invention is operable with any water soluble silicate, but those of the alkali metals are preferred, mainly those of sodium and potassium.

If the insoluble silicate is separately prepared, the general method of practicing this feature of the invention is as follows:

The insoluble silicate is preferably prepared by a double decomposition reaction in aqueous media between a soluble silicate and the salt of a metal, the silicate of which is substantially insoluble.

It will be found that the resulting'precipitated silicate will be gelatinous, hydrated, and noncoagulated. For optimum results care should be exercised that the pigment be incorporated with the gelatinous silicate before the silicate dehydrates and dries. This incorporation of Pigment particles with the gelatinous silicate may b affected. by means of any convenient mixing technique, for instance, millin grinding, vigorous agitation, and so forth.

The amount of precipitated silicate incorporated in the pigment is in no way restricted to any specific amount, but is determined to a large extent in each individual case by the special use for which the pigment is intended. However, in general, the amount of silicate associated with the titanium dioxide pigment for most practical purposes restricts itself to between a fraction of 1.0 percent and about 10.0 percent. For ordinary use about 2.0 percent to 5.0 percent of that the invention thereby should not be limited p or restricted in any way-or manner. a Example 1 1000 parts of calcined titanium dioxide are slurried in 3760 parts of water containing 28.3 parts of MgClz. Thorough distribution of the salt in the slurry is insured by adequate stirring.

To this slurry is now slowly added during conthe silicate has been found to be suflicient'for a marked improvement in resistance to discoloration and weathering, at the same time not disturbing other pigment characteristics to any noticeable degree. When the silicate is formed, in situ, the amount thereof desired will be obtained by controlling the amount of water solwatered, dried, and subjected to a light pulverizing treatment. The treated pigment is then ready for use in surface coating compositions.

It will be understood-in connection with the practice of invention that the various silicates such as those of aluminum and zirconium may differ somewhat in degree of solubility and in tendency to hydrolyze. For example, the silicates of cadmium and aluminum are very slightly soluble in water, but nevertheless useful in the practice of the present invention. The silicate of barium exhibits some solubility as well as a tendency to hydrolyze. Accordingly, when intimately associating titanium dioxide pigment particles with barium, it is recommended that the operation be carried out in the cold. Ordinarily, however, the invention may be practiced at elevated temperatures if desired, and in fact, it appears that the intimate association of silicate and pigment particles is favored by proceeding at temperatures above normal, for example, between 40 C. and the boiling point of the solution, preferably between 60 C. and 80 C. When operated at the elevated temperatures, it appears that the silicate more uniformly and evenly associates itself with the titanium particles. While the nature of the association is not known, it may be that the silicate coats the particles or perhaps penetrates into the structure of the particles. Furthermore, it has been found that good results may be obtained by leaving the treated pigment in contact with the reaction mixture after formation of the insoluble silicate, or after mixing the pigment particles with a separately prepared insoluble silicate gel. By thus permitting the pigment particles to remain in contact with the liquid medium, the association of the pigment particles and the silicate appears to be further favored. This retention period may be for as little as one hour, or if desired, for as long as twenty-four hours.

In order that the invention may be more readily and thoroughly comprehended a few examples of the process which has been described above are given below, it being understood that this is done for purely illustrative purposes and stant agitation a solution of sodium metasilicate containing 36.3 parts of NazSiOa, an amount chemically equivalent to the amount of magnesium chloride present in the slurry. At the conclusion of the addition of the sodium silicate the pH value of the slurry is adjusted to between 7.0 and 7.4 and the slurry is then filtered. The treated pigment is washed to remove sodium chloride and is finally dried and disintegrated before being used.

Other soluble salts of the second group metals such as zinc sulphate can be substituted for the magnesium chloride used in this example.

Example II I 1000 parts of calcined titanium dioxide are slurried in 3760 parts of water and treated with a sodium silicate solution containing 36.3 parts of Na2SiO3 in 300 parts of water. Thorough distribution of the silicate in the slurry is insured by adequate agitation. To this slurry wherei'rom, if so desired, all gritty materials and large agglomerates now may be eliminated by known means of hydroseparation before further treatment, is slowly added a solution of magnesium chloride containing 28.3 parts of MgClz in '300 parts of water, an amount chemically equivalent to the amount of sodium silicate present. The slurry is thoroughly agitated during the addition of the magnesium chloride and is, after the conclusion of the treatment and the adjustment to the proper pH, filtered and washed to remove sodium chloride, the pigment finally being dried and disintegrated before being used.

Other soluble salts of the second group metals such as, for example, zinc sulfate, can be substituted for the magnesium chloride used in this example.

Example III 1000 parts of calcined titanium dioxide are slurried in 3760 parts of water containing 50 grams of A1C13.6H2O. To this slurry, which is being thoroughly agitated to insure complete distribution of the aluminum salt, is now slowly added a solution of Metso (NazSiOafiHzO) equal to 37.9 parts of NazSiOa in 300 parts of water, an amount chemically equivalent to the amount of aluminum chloride present. conclusion of the treatment, when the double decomposition of the aluminum chloride and the sodium silicate is completed, the pH of the slurry is adjusted to the proper value, 7.0 to 7.4, and the pigment washed to remove sodium chloride, whereupon-the pigment is dried at C. and disintegrated before being used.

Other soluble salts of the third group metals, such as, for example, aluminum sulfate and yttrium chloride can be substituted for the aluminum chloride used in the above example.

Example IV 900 parts of calcined titanium dioxide are slurried in 3630 parts of water containing 45 parts of ZrOC12.8H2O. The slurry is thoroughly agitated to insure complete distribution of the zirconium salt, and a solution of sodium silicate containing At the 30 parts of Metso (NazSiOaHaO) in 300 parts of water, an amount of silicate chemically equivalent to the amountof zirconium salt present. that is for the formation of the zirconium silicate (ZIO.S102) is slowly added. The slurry is, after 5 adjustment to the proper pH, filtered and washed to remove sodium chloride and the pigment is finally dried and finished in the usual manner.

Other soluble salts of the metals of the fourth group can be used for the treatment, such as the sulfate of zirconium, cerium salts and others.

In order to test the pigments which are described in the foregoing examples as to their weathering characteristics. an exterior paint enamel was formulated in which the silicatetreated pigments each were incorporated as the single pigmenting material except for a small amount of lamp black which was added as a tint-- ing agent. The prepared panels. together with the proper'control (untreated pigments) were exposed outdoors.

The cedar panels were coated with three coats of enamels pigmented with 100 percent titanium dioxide pigment in a vehicle consisting of:

Percent Heat processed tung linseed oil 14.9 Heat bodied linseed oil, acid No. 2.4; Gardner-Holdt viscosity R to S 21.18 Phenolic resin varnish 40.93 Thinner and driers 22.95

The pigment was approximately 20.3% by volume of the total non-volatile.

N oras.--None-no chalking V. sl.-very slight chalking Sl.-slight chalking Cons-considerable chalking B-bad chalking V. B.=Very bad chalking As an example of the improved resistance to discoloration on exposure to light, which is obtained in pigments which have been treated with relatively insoluble silicates, may be given the 5 titanium dioxide pigment containing 3.0 percent zirconiumsilicate described in Example IV.

This pigment was tested in a dammar solution and in a Bakelite resin varnish under ultra-violet light. The silicate-treated pigment was in both cases superior to the untreated pigment in regar to resistance to discoloration.

The results are given below.

Discolorationin as lite e (pinkish) Damar Control High.... 2 1. Example IV,3.0% Zr silicate. do. 1+ Better than i.

The degree of discoloration is expressed in relation to arbitrarily chosen reflectances, where the numeral 1 designates the value of the best standard pigment and the numeral 2 the value of a diflerent and inferior standard. For the socalled Bakelite test the pigment was formulated in an exterior paint using 100 percent titanium dioxide as a pigment. The vehicle consisted of:

, Percent Heat-processed tung linseed oil 15.5 Heat-bodied linseed oil 22.1 Bakelite resin varnish (phenol formaldehyde type) 42.8 Mineral spirits 17.6 Cobalt drier cooked in 20 The pigment was 50 percent by weight of total non-volatile (vehicle and pigment).

For the dammar test the pigment was formulated in a dammar solution consisting of 50 percent dammar and 50 percent'turpentine, using 70% by weight of dammar solution and 30% by weight of 100 percent titanium dioxide pigment.

The foregoing examples which have been described in detail for the purpose of making the invention more easily understood have been limited to a commercially pure titanium dioxide pig ment, but the silicate treatment may naturally be applied with equally as great an advantage to composite titanium dioxide pigments which contain extenders such as BaSO4 or CaSOr or others, and to titanates and other pigments containing titanium. It is also understood that the inven-- tion is not limited to the use of soluble metasili-- cates but that other soluble silicates with other mol ratios of alkali to silicic acid than that of the metasilicate can equally as well be used in the treatment.

I claim:

1. A process for treating a titanium dioxide pigment which comprises mixing with an aqueous slurry of said pigment an alkali-metal silicate, and thereafter adding to the slurry as a 40 precipitant a water-soluble salt of a second group metal thereby precipitating an insoluble silicate in the presence of the pigment.

2. An improved pigment comprising titanium dioxide and the reaction product of an alkali metal silicate and a water-soluble salt of a second group metal capable o1 precipitating the silicate in insoluble form.

3. An improved pigment comprising a base pigment containing titanium dioxide, and the reaction product of an alkali metal silicate and a soluble salt capable of precipitating the silicate in insoluble form, the amount of said reaction product being suiiicient to improve the pigment properties ofthe pigment.

4. A vprocess for treating a titanium dioxide pigment which comprises'mixing with an aqueous slurry of said pigment, a soluble silicate and a soluble salt capable of precipitating the silicate in insoluble form.

5. A process for treating a titanium dioxide pigment which comprises mixing withan aqueous slurry of said pigment a soluble silicate and thereafter adding to the mixture as a precipitant a soluble salt capable of precipitating aninsoluble silicate in the presence of the pigment.

6. A process for treating a titanium dioxide pigment which comprises mixing with an aqueous slurry of said pigment a soluble salt of a metal, the'silicate of which is substantially insoluble in water, and thereafter adding to the mixture a soluble silicate thereby precipitating an insoluble silicate in the presence of the pigment.

7. A process for treating a titanium dioxide pigment which comprises mixing with an aquea soluble salt of a second-group metal capable of precipitating the silicate in insoluble form.

8. A process for treating a titanium dioxide pigment which comprises mixing with an aqueous slurry of said pigment a soluble silicate and a soluble salt of a fourth-group metal capable of precipitating the silicate in insoluble form.

9. A process for treating a titanium dioxide pigment which comprises mixing with an aqueous slurry oi. said pigment a soluble salt of zirconium and thereafter adding to the mixture an alkali metal silicate in an amount between about 1% and about 10%, based on the weight or the pigment, thereby precipitating zirconium silicate in the presence 01 the pigment.

10. An improved pigment comprising titanium dioxide and the reaction product 01 a soluble 11. An improved pigment comprising titanium ing the silicate in insoluble form.

dioxide and the reaction product 01 a soluble sillcate and a soluble salt of a second-group metal capable of precipitating the silicate in insoluble form.

12. An improved pigment comprising titanium dioxide and the reaction product of a soluble silicate and a soluble salt of a fourth-group metal capable of precipitating the silicate in insoluble form.

13. An improved pigment comprising titanium dioxide and the reaction product of an alkali metal silicate and a soluble salt of zirconium.

14. A pigmented surface coating composition the pigment portion of which comprises a tita-' mum dioxide pigment intimately associated with a small amount oi. the reaction product 01 a soluble silicate and a soluble salt capable of precipitating the silicate in insoluble form.

- DURANT W. ROBERTSON. 

